I. Field of the Invention
The present invention relates to aircraft turbine engines, and more particularly to a means for varying the cooling airflow in such engines, for example, the cooling airflow impinging on the turbine blades.
II. Description of the Prior Art
Improvements in aircraft turbine engine performance typically have been achieved by increasing the gas temperature at the inlet of the turbine rotor. Such temperatures have now been increased to the point that several current engines operate at turbine entry temperatures levels which exceed the melting points of the materials of which the turbines are composed. Therefore, means have been required for cooling these materials to sufficiently low temperatures to render them mechanically stable and otherwise within an acceptable range.
The known means for cooling the turbine component often include means for routing cooling air to the turbine components, such as to the turbine blades, in order to provide the required cooling. This cooling air is usually a portion of the intake air provided to the turbine by one of the compressor stages. The cooling air is diverted from the outlet of the compressor stage, bypassing the engine combustor, and is then allowed to pass along or through the turbine components to be cooled.
While the prior structures for providing turbine cooling air have been adequate to cool the turbine components sufficiently to permit the employment of the desired elevated combustion temperatures, such structures have generally been subject to the disadvantage that they have allowed bypass or cooling air to impinge upon the turbine components even during those circumstances when such impingement is unnecessary, for example, when the turbine entry temperatures are sufficiently low so that any cooling of the turbine components is unnecessary. Such improper impingement can be deleterious to engine performance, such as when the engine is operated at low power.
This drawback has been only partially met by prior devices for modulating the flow of the cooling air. For example, as disclosed in U.S. Pat. No. 4,416,111, some prior devices have involved the use of a pivotable valve door cooperating with part of a vaned, annular conduit (such as with the turbine nozzle) to block only a portion of the conduit, so as to reduce the cooling airflow to the turbine section of the engine. Devices of this are disadvantageous for two reasons, however. First, the flow of cooling air is merely reduced and not eliminated. Second, such devices have been relatively complex, thereby being costly to construct yet readily subject to failure in use.